Forward converter having a primary-side current sense circuit

ABSTRACT

A load control device for controlling the amount of power delivered to an electrical load (e.g., an LED light source) includes first and second semiconductor switches, a transformer, a capacitor, a controller, and a current sense circuit operable to receive a sense voltage representative of a primary current conducting through to a primary winding of the transformer. The primary winding is coupled in series with a semiconductor switch, while a secondary winding is adapted to be operatively coupled to the load. The capacitor is electrically coupled between the junction of the first and second semiconductor switches and the primary winding. The current sense circuit receives a sense voltage and averages the sense voltage when the first semiconductor switch is conductive, so as to generate a load current control signal that is representative of a real component of a load current conducted through the load.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/584,758, filed May 2, 2017, which issued as U.S. Pat. No. 10,219,335on Feb. 26, 2019, which is a continuation of U.S. patent applicationSer. No. 14/940,540, filed Nov. 13, 2015, which issued as U.S. Pat. No.9,655,177 on May 16, 2017, which is a continuation of U.S. patentapplication Ser. No. 13/834,153, filed Mar. 15, 2013, which issued asU.S. Pat. No. 9,232,574 on Jan. 5, 2016, which claims the benefit ofcommonly-assigned U.S. Provisional Application No. 61/668,759, filedJul. 6, 2012, entitled LOAD CONTROL DEVICE FOR A LIGHT-EMITTING DIODELIGHT SOURCE, the entire disclosures of which are hereby incorporated byreference.

BACKGROUND

Light-emitting diode (LED) light sources (i.e., LED light engines) areoften used in place of or as replacements for conventional incandescent,fluorescent, or halogen lamps, and the like. LED light sources maycomprise a plurality of light-emitting diodes mounted on a singlestructure and provided in a suitable housing. LED light sources aretypically more efficient and provide longer operational lives ascompared to incandescent, fluorescent, and halogen lamps. In order toilluminate properly, an LED driver control device (i.e., an LED driver)must be coupled between an alternating-current (AC) source and the LEDlight source for regulating the power supplied to the LED light source.The LED driver may regulate either the voltage provided to the LED lightsource to a particular value, the current supplied to the LED lightsource to a specific peak current value, or may regulate both thecurrent and voltage.

LED light sources are typically rated to be driven via one of twodifferent control techniques: a current load control technique or avoltage load control technique. An LED light source that is rated forthe current load control technique is also characterized by a ratedcurrent (e.g., approximately 350 milliamps) to which the peak magnitudeof the current through the LED light source should be regulated toensure that the LED light source is illuminated to the appropriateintensity and color. In contrast, an LED light source that is rated forthe voltage load control technique is characterized by a rated voltage(e.g., approximately 15 volts) to which the voltage across the LED lightsource should be regulated to ensure proper operation of the LED lightsource. Typically, each string of LEDs in an LED light source rated forthe voltage load control technique includes a current balance regulationelement to ensure that each of the parallel legs has the same impedanceso that the same current is drawn in each parallel string.

It is known that the light output of an LED light source can be dimmed.Different methods of dimming LEDs include a pulse-width modulation (PWM)technique and a constant current reduction (CCR) technique. Pulse-widthmodulation dimming can be used for LED light sources that are controlledin either a current or voltage load control mode. In pulse-widthmodulation dimming, a pulsed signal with a varying duty cycle issupplied to the LED light source. If an LED light source is beingcontrolled using the current load control technique, the peak currentsupplied to the LED light source is kept constant during an on time ofthe duty cycle of the pulsed signal. However, as the duty cycle of thepulsed signal varies, the average current supplied to the LED lightsource also varies, thereby varying the intensity of the light output ofthe LED light source. If the LED light source is being controlled usingthe voltage load control technique, the voltage supplied to the LEDlight source is kept constant during the on time of the duty cycle ofthe pulsed signal in order to achieve the desired target voltage level,and the duty cycle of the load voltage is varied in order to adjust theintensity of the light output. Constant current reduction dimming istypically only used when an LED light source is being controlled usingthe current load control technique. In constant current reductiondimming, current is continuously provided to the LED light source,however, the DC magnitude of the current provided to the LED lightsource is varied to thus adjust the intensity of the light output.Examples of LED drivers are described in greater detail incommonly-assigned U.S. patent application Ser. No. 12/813,908, filedJun. 11, 2010, and U.S. patent application Ser. No. 13/416,741, filedMar. 9, 2012, both entitled LOAD CONTROL DEVICE FOR A LIGHT-EMITTINGDIODE LIGHT SOURCE, the entire disclosures of which are herebyincorporated by reference.

In addition, some LED light sources comprise forward converters fordriving the LED light sources to control the load current conductedthrough the LED light source. Forward converters comprise a transformerhaving a primary winding coupled to at least one semiconductor switchand a secondary winding operatively coupled to the LED light source. Thesemiconductor switch is rendered conductive and non-conductive toconduct a primary current through the primary winding and to thustransfer power to the secondary winding of the transformer. Forwardconverters typically comprise an optocoupler for coupling a feedbacksignal on the secondary side of the transformer to the primary side ofthe transformer, such that a controller can control the semiconductorswitch is response to the feedback signal. However, there is a need fora forward converter that is able to control the magnitude of the loadcurrent through an LED light source without the need for an optocoupler.

SUMMARY

The present disclosure relates to a load control device for anelectrical load, such as a light-emitting diode (LED) driver forcontrolling the intensity of an LED light source.

As described herein, a load control device for controlling the amount ofpower delivered to an electrical load may include first and secondsemiconductor switches, a transformer, a capacitor, a controller, and acurrent sense circuit. The first and second semiconductor switcheselectrically coupled in series and configured to be controlled togenerate an inverter voltage at a junction of the first and secondsemiconductor switches. The transformer may include a primary windingcoupled between circuit common and the junction of the first and secondsemiconductor switches. The transformer may include a secondary windingadapted to supply current to the electrical load. For example, thetransformer may be configured to transfer power to the secondary windingwhen either of the first and second semiconductor switches isconductive. The first and second semiconductor switches and thetransformer may be part of an isolated forward converter. The convertermay be configured to receive a bus voltage and to conduct a load currentthrough the electrical load.

The capacitor may be electrically coupled between the junction of thefirst and second semiconductor switches and the primary winding of thetransformer to cause a primary voltage across the primary winding tohave a positive polarity when the first semiconductor switch isconductive and a negative polarity when the second semiconductor switchis conductive. The controller may be configured to control the firstsemiconductor switch to control a load current conducted through theelectrical load. The controller may be further configured to control theamount of power delivered to the electrical load to a target amount ofpower.

The current sense circuit may be configured to receive a sense voltagerepresentative of a magnitude of a primary current conducted through theprimary winding. The current sense circuit may include an averagingcircuit configured to average the sense voltage when the firstsemiconductor switch of the isolated forward converter is conductive togenerate a load current control signal that is representative of a realcomponent of the primary current. The current sense circuit may beconfigured to average the sense voltage for an on time when the firstsemiconductor switch of the isolated forward converter is conductiveplus an additional amount of time to generate a load current controlsignal that is representative of a real component of the primarycurrent. The additional amount of time may be included when the targetamount of power described herein is less than a threshold amount. Theduration of the additional amount of time may be a function of thetarget amount of power (e.g., the additional amount of time may increaselinearly as the target amount of power decreases).

An LED driver for controlling the intensity of an LED light source isalso described herein. The LED driver may include a transformer, acontroller, and a current sense circuit. The transformer may include aprimary winding and a secondary winding adapted to supply current to theLED light source. The controller may be configured to control a loadcurrent conducted through the LED light source to control the intensityof the LED light source to a target intensity. The LED driver may alsoinclude an isolated forward converter that may be configured to receivea bus voltage and to conduct a load current through the LED lightsource. The isolated forward converter may include the transformer and ahalf-bridge inverter circuit for generating an inverter voltage. Thehalf-bridge inverter circuit may be coupled to the primary winding ofthe transformer through a capacitor to produce a primary voltage acrossthe primary winding. The controller may be configured to control thehalf-bridge inverter circuit of the isolated forward converter so thatthe load current conducted through the LED light source may becontrolled. The intensity of the LED light source may also be controlledto reach a target intensity. The current sense circuit may be configuredto receive a sense voltage representative of a magnitude of a primarycurrent conducted through the primary winding. The current sense circuitmay be further configured to average the sense voltage when themagnitude of the primary voltage across the primary winding is positiveand greater than approximately zero volts. A load current control signalthat is representative of a real component of the primary current may begenerated as a result.

Also described herein is a forward converter for controlling the amountof power delivered to an electrical load from an input voltage. Theforward converter may include a transformer, a half-bridge invertercircuit, a capacitor, a controller, and a current sense circuit. Thetransformer may include a primary winding and a secondary windingadapted to supply current to the electrical load. The half-bridgeinverter circuit may include first and second semiconductor switchescoupled in series across the input voltage and configured to generate aninverter voltage at a junction of the two semiconductor switches. Thecapacitor may be coupled between the junction of the two semiconductorswitches and the primary winding of the transformer such that a primaryvoltage may be produced across the primary winding. The transformer maybe further configured to transfer power to the secondary winding wheneither of the semiconductor switches is conductive. The controller mayconfigured to control the first and second semiconductor switches sothat a load current conducted through the electrical load may becontrolled. The current sense circuit may be configured to receive asense voltage representative of a magnitude of a primary currentconducted through the primary winding. The current sense circuit may beconfigured to average the sense voltage when the first semiconductorswitch of the half-bridge inverter circuit is conductive. A load currentcontrol signal that is representative of a real component of the loadcurrent may be generated as a result.

Other features and advantages of the present invention will becomeapparent from the following description of the invention that refers tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a light-emitting diode (LED)driver for controlling the intensity of an LED light source.

FIG. 2 is a simplified schematic diagram of an isolated forwardconverter and a current sense circuit of an LED driver.

FIG. 3 is an example diagram illustrating a magnetic core set of anenergy-storage inductor of a forward converter.

FIG. 4 shows example waveforms illustrating the operation of a forwardconverter and a current sense circuit when the intensity of an LED lightsource is near a high-end intensity.

FIG. 5 shows example waveforms illustrating the operation of a forwardconverter and a current sense circuit when the intensity of an LED lightsource is near a low-end intensity.

FIG. 6 is an example plot of a relationship between an offset time and atarget intensity of an LED driver.

FIG. 7 is a simplified flowchart of a control procedure executedperiodically by a controller of an LED driver.

FIG. 8 is an example plot of a relationship between the offset time andthe target intensity of an LED driver.

FIG. 9 shows an example waveform of a load current conducted through anLED light source when a target current of an LED driver is at asteady-state value.

FIG. 10 shows an example waveform of the load current conducted throughthe LED light source when the target current of the LED driver is beingincreased with respect to time.

FIG. 11 shows example waveforms of a ramp signal of an LED driver and aload current conducted through an LED light source when the ramp signalis added to a target current.

DETAILED DESCRIPTION

FIG. 1 is a simplified block diagram of a light-emitting diode (LED)driver 100 for controlling the intensity of an LED light source 102(e.g., an LED light engine). The LED light source 102 is shown as aplurality of LEDs connected in series but may comprise a single LED or aplurality of LEDs connected in parallel or a suitable combinationthereof, depending on the particular lighting system. In addition, theLED light source 102 may alternatively comprise one or more organiclight-emitting diodes (OLEDs). The LED driver 100 comprises a hotterminal H and a neutral terminal that are adapted to be coupled to analternating-current (AC) power source (not shown).

The LED driver 100 comprises a radio-frequency (RFI) filter circuit 110for minimizing the noise provided on the AC mains and a rectifiercircuit 120 for generating a rectified voltage V_(RECT). The LED driver100 further comprises a boost converter 130, which receives therectified voltage V_(RECT) and generates a boosted direct-current (DC)bus voltage V_(BUS) across a bus capacitor C_(BUS). The boost converter130 may alternatively comprise any suitable power converter circuit forgenerating an appropriate bus voltage, such as, for example, a flybackconverter, a single-ended primary-inductor converter (SEPIC), a aukconverter, or other suitable power converter circuit. The boostconverter 120 may also operate as a power factor correction (PFC)circuit to adjust the power factor of the LED driver 100 towards a powerfactor of one. The LED driver 100 also comprises an isolated,half-bridge forward converter 140, which receives the bus voltageV_(BUS) and controls the amount of power delivered to the LED lightsource 102 so as to control the intensity of the LED light sourcebetween a low-end (i.e., minimum) intensity L_(LE) (e.g., approximately1-5%) and a high-end (i.e., maximum) intensity L_(HE) (e.g.,approximately 100%).

The LED driver 100 further comprises a control circuit, e.g., acontroller 150, for controlling the operation of the boost converter 130and the forward converter 140. The controller 150 may comprise, forexample, a digital controller or any other suitable processing device,such as, for example, a microcontroller, a programmable logic device(PLD), a microprocessor, an application specific integrated circuit(ASIC), or a field-programmable gate array (FPGA). The controller 150generates a bus voltage control signal V_(BUS-CNTL), which is providedto the boost converter 130 for adjusting the magnitude of the busvoltage V_(BUS). The controller 150 receives from the boost converter130 a bus voltage feedback control signals V_(BUS-FB), which isrepresentative of the magnitude of the bus voltage V_(BUS).

The controller 150 also generates drive control signals V_(DRIVE1),V_(DRIVE2), which are provided to the forward converter 140 foradjusting the magnitude of a load voltage V_(LOAD) generated across theLED light source 102 and the magnitude of a load current I_(LOAD)conducted through the LED light source to thus control the intensity ofthe LED light source to a target intensity L_(TRGT). The LED driver 100further comprises a current sense circuit 160, which is responsive to asense voltage V_(SENSE) that is generated by the forward converter 140and is representative of the magnitude of the load current I_(LOAD). Thecurrent sense circuit 160 is responsive to a signal-chopper controlsignal V_(CHOP) (which is received from the controller 150), andgenerates a load current feedback signal V_(I-LOAD) (which is a DCvoltage representative of the magnitude of the load current I_(LOAD)).The controller 150 receives the load current feedback signal V_(I-LOAD)from the current sense circuit 160 and controls the drive controlsignals V_(DRIVE1), V_(DRIVE2) to adjust the magnitude of the loadcurrent LOAD to a target load current I_(TRGT) to thus control theintensity of the LED light source 102 to the target intensity L_(TRGT).The target load current I_(TRGT) may be adjusted between a minimum loadcurrent I_(MIN) and a maximum load current I_(MAX).

The controller 150 is coupled to a memory 170 for storing theoperational characteristics of the LED driver 100 (e.g., the targetintensity L_(TRGT), the low-end intensity L_(LE), the high-end intensityL_(HE), etc.). The LED driver 100 may also comprise a communicationcircuit 180, which may be coupled to, for example, a wired communicationlink or a wireless communication link, such as a radio-frequency (RF)communication link or an infrared (IR) communication link. Thecontroller 150 may be operable to update the target intensity L_(TRGT)of the LED light source 102 or the operational characteristics stored inthe memory 170 in response to digital messages received via thecommunication circuit 180. Alternatively, the LED driver 100 could beoperable to receive a phase-control signal from a dimmer switch fordetermining the target intensity L_(TRGT) for the LED light source 102.The LED driver 100 further comprises a power supply 190, which receivesthe rectified voltage V_(RECT) and generates a direct-current (DC)supply voltage V_(CC) for powering the circuitry of the LED driver.

FIG. 2 is a simplified schematic diagram of a forward converter 240 anda current sense circuit 260, e.g., the forward converter 140 and thecurrent sense circuit 160 of the LED driver 100 shown in FIG. 1. Theforward converter 240 comprises a half-bridge inverter circuit havingtwo field effect transistors (FETs) Q210, Q212 for generating ahigh-frequency inverter voltage V_(INV) from the bus voltage V_(BUS).The FETs Q210, Q212 are rendered conductive and non-conductive inresponse to the drive control signals V_(DRIVE1), V_(DRIVE2), which arereceived from a controller (e.g., the controller 150). The drive controlsignals V_(DRIVE1), V_(DRIVE2) are coupled to the gates of therespective FETs Q210, Q212 via a gate drive circuit 214 (e.g., partnumber L6382DTR, manufactured by ST Microelectronics). The controllergenerates the inverter voltage V_(INV) at a constant operating frequencyf_(OP) (e.g., approximately 60-65 kHz) and thus a constant operatingperiod T_(OP). However, the operating frequency f_(OP) may be adjustedunder certain operating conditions. The controller adjusts the dutycycle DC of the inverter voltage V_(INV) to adjust the magnitude of theload current LOAD and thus the intensity of an LED light source 202. Theforward converter 240 may be characterized by a turn-on time T_(TURN-ON)from when the drive control signals V_(DRIVE1), V_(DRIVE2) are drivenhigh until the respective FET Q210, Q212 is rendered conductive, and aturn-off time T_(TURN-OFF) from when the drive control signalsV_(DRIVE1), V_(DRIVE2) are driven low until the respective FET Q210,Q212 is rendered non-conductive.

The inverter voltage V_(INV) is coupled to the primary winding of atransformer 220 through a DC-blocking capacitor C216 (e.g., having acapacitance of approximately 0.047 μF), such that a primary voltageV_(PRI) is generated across the primary winding. The transformer 220 ischaracterized by a turns ratio n_(TURNS) (i.e., N₁/N₂) of approximately115:29. The sense voltage V_(SENSE) is generated across a sense resistorR222, which is coupled series with the primary winding of thetransformer 220. The FETs Q210, Q212 and the primary winding of thetransformer 220 are characterized by parasitic capacitances C_(P1),C_(P2), C_(P3).

The secondary winding of the transformer 220 generates a secondaryvoltage, which is coupled to the AC terminals of a full-wave dioderectifier bridge 224 for rectifying the secondary voltage generatedacross the secondary winding. The positive DC terminal of the rectifierbridge 224 is coupled to the LED light source 202 through an outputenergy-storage inductor L226 (e.g., having an inductance ofapproximately 10 mH), such that the load voltage V_(LOAD) is generatedacross an output capacitor C228 (e.g., having a capacitance ofapproximately 3 μF).

FIG. 3 is an example diagram illustrating a magnetic core set 290 of anenergy-storage inductor (e.g., the output energy-storage inductor L226of the forward converter 240 shown in FIG. 2). The magnetic core set 290comprises two E-cores 292A, 292B, and may comprise part numberPC40EE16-Z, manufactured by TDK Corporation. The E-cores 292A haverespective outer legs 294A, 294B and inner legs 296A, 296B. Each innerleg 296A, 296B may have a width w_(LEG) (e.g., approximately 4 mm). Theinner leg 296A of the first E-core 292A has a partial gap 298A (i.e.,the magnetic core set 290 is partially-gapped), such that the inner legs296A, 296B are spaced apart by a gap distance d_(GAP) (e.g.,approximately 0.5 mm). The partial gap 298A may extend for a gap widthw_(GAP), e.g., approximately 2.8 mm, such that the gap extends forapproximately 70% of the leg width w_(LEG) of the inner leg 296A.Alternatively, both of the inner legs 296A, 296B could comprise partialgaps. The partially-gapped magnetic core set 290 shown in FIG. 3 allowsthe output energy-storage inductor L226 of the forward converter 240shown in FIG. 2 to maintain continuous current at low load conditions(e.g., near the low-end intensity L_(LE)).

FIG. 4 shows example waveforms illustrating the operation of a forwardconverter and a current sense circuit, e.g., the forward converter 240and the current sense circuit 260 shown in FIG. 2. The controller drivesthe respective drive control signals V_(DRIVE1), V_(DRIVE2) high toapproximately the supply voltage V_(CC) to render the respective FETsQ210, Q212 conductive for an on time T_(ON) at different times (i.e.,the FETs Q210, Q212 are not conductive at the same time). When thehigh-side FET Q210 is conductive, the primary winding of the transformer220 conducts a primary current I_(PRI) to circuit common through thecapacitor C216 and sense resistor R222. Immediately after the high-sideFET Q210 is rendered conductive (at time t₁ in FIG. 4), the primarycurrent I_(PRI) conducts a short high-magnitude pulse of current due tothe parasitic capacitance C_(P3) of the transformer 220 as shown in FIG.4. While the high-side FET Q210 is conductive, the capacitor C216charges, such that a voltage having a magnitude of approximately half ofthe magnitude of the bus voltage V_(BUS) is developed across thecapacitor. Accordingly, the magnitude of the primary voltage V_(PRI)across the primary winding of the transformer 220 is approximately equalto approximately half of the magnitude of the bus voltage V_(BUS). Whenthe low-side FET Q212 is conductive, the primary winding of thetransformer 220 conducts the primary current I_(PRI) in an oppositedirection and the capacitor C216 is coupled across the primary winding,such that the primary voltage V_(PRI) has a negative polarity with amagnitude equal to approximately half of the magnitude of the busvoltage V_(BUS).

When either of the high-side and low-side FETs Q210, Q212 areconductive, the magnitude of an output inductor current I_(L) conductedby the output inductor L226 and the magnitude of the load voltageV_(LOAD) across the LED light source 202 both increase with respect totime. The magnitude of the primary current I_(PRI) also increases withrespect to time while the FETs Q210, Q212 are conductive (after theinitial current spike). When the FETs Q210, Q212 are non-conductive, theoutput inductor current I_(L) and the load voltage V_(LOAD) bothdecrease in magnitude with respective to time. The output inductorcurrent I_(L) is characterized by a peak magnitude I_(L-PK) and anaverage magnitude I_(L-AVG) as shown in FIG. 4. The controller increasesand decreases the on times T_(ON) of the drive control signalsV_(DRIVE1), V_(DRIVE2) (and the duty cycle DC of the inverter voltageV_(INV)) to respectively increase and decrease the average magnitudeI_(L-AVG) of the output inductor current I_(L) and thus respectivelyincrease and decrease the intensity of the LED light source 102.

When the FETs Q210, Q212 are rendered non-conductive, the magnitude ofthe primary current I_(PRI) drops toward zero amps (e.g., as shown attime t₂ in FIG. 4 when the high-side FET Q210 is renderednon-conductive). However, current may continue to flow through theprimary winding of the transformer 220 due to the magnetizing inductanceL_(MAG) of the transformer (which conducts a magnetizing currentI_(MAG)). In addition, when the target intensity L_(TRGT) of the LEDlight source 102 is near the low-end intensity L_(LE), the magnitude ofthe primary current I_(PRI) oscillates after either of the FETs Q210,Q212 is rendered non-conductive due to the parasitic capacitancesC_(P1), C_(P2) of the FETs, the parasitic capacitance C_(P3) of theprimary winding of the transformer 220, and any other parasiticcapacitances of the circuit, such as, parasitic capacitances of theprinted circuit board on which the forward converter 240 is mounted.

The real component of the primary current I_(PRI) is representative ofthe magnitude of the secondary current I_(SEC) and thus the intensity ofthe LED light source 202. However, the magnetizing current I_(MAG)(i.e., the reactive component of the primary current I_(PRI)) also flowsthrough the sense resistor R222. The magnetizing current I_(MAG) changesfrom negative to positive polarity when the high-side FET Q210 isconductive, changes from positive to negative polarity when the low-sideFET Q212 is conductive, and remains constant when the magnitude of theprimary voltage V_(PRI) is zero volts as shown in FIG. 4. Themagnetizing current I_(MAG) has a maximum magnitude defined by thefollowing equation:

$\begin{matrix}{I_{{MAG}\text{-}{MAX}} = {\frac{V_{BUS} \cdot T_{HC}}{4 \cdot L_{MAG}}.}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$where T_(HC) is the half-cycle period of the inverter voltage V_(INV),i.e., T_(HC)=T_(OP)/2. As shown in FIG. 4, the areas 250, 252 areapproximately equal, such that the average value of the magnitude of themagnetizing current I_(MAG) when the magnitude of the primary voltageV_(PRI) is greater than approximately zero volts.

The current sense circuit 260 averages the primary current I_(PRI)during the positive cycles of the inverter voltage V_(INV), i.e., whenthe high-side FET Q210 is conductive. The load current feedback signalV_(I-LOAD) generated by the current sense circuit 260 has a DC magnitudethat is the average value of the primary current I_(PRI) when thehigh-side FET Q210 is conductive. Because the average value of themagnitude of the magnetizing current I_(MAG) is approximately zero whenthe high-side FET Q210 is conductive, the load current feedback signalV_(I-LOAD) generated by the current sense circuit is representative ofonly the real component of the primary current I_(PRI).

The current sense circuit 260 comprises an averaging circuit forproducing the load current feedback signal V_(I-LOAD). The averagingcircuit may comprise a low-pass filter having a capacitor C230 (e.g.,having a capacitance of approximately 0.066 uF) and a resistor R232(e.g., having a resistance of approximately 3.32 kΩ). The low-passfilter receives the sense voltage V_(SENSE) via a resistor R234 (e.g.,having resistances of approximately 1 kΩ). The current sense circuit 160further comprises a transistor Q236 (e.g., a FET as shown in FIG. 2)coupled between the junction of the resistors R232, R234 and circuitcommon. The gate of the transistor Q236 is coupled to circuit commonthrough a resistor R238 (e.g., having a resistance of approximately 22kΩ) and receives the signal-chopper control signal V_(CHOP) from thecontroller.

When the high-side FET Q210 is rendered conductive, the controllerdrives the signal-chopper control signal V_(CHOP) low towards circuitcommon to render the transistor Q236 non-conductive for a signal-choppertime T_(CHOP), which is approximately equal to the on time T_(ON) of thehigh-side FET Q210 as shown in FIG. 4. The capacitor C230 is able tocharge from the sense voltage V_(SENSE) through the resistors R232, R234while the signal-chopper control signal V_(CHOP) is low, such that themagnitude of the load current feedback signal V_(I-LOAD) is the averagevalue of the primary current I_(PRI) and is thus representative of thereal component of the primary current during the time when the high-sideFET Q210 is conductive. When the high-side FET Q210 is not conductive,the controller 150 drives the signal-chopper control signal V_(CHOP)high to render the transistor Q236 non-conductive. Accordingly, thecontroller is able to accurately determine the average magnitude of theload current LOAD from the magnitude of the load current feedback signalV_(I-LOAD) since the effects of the magnetizing current I_(MAG) and theoscillations of the primary current I_(PRI) on the magnitude of the loadcurrent feedback signal V_(I-LOAD) are reduced or eliminated completely.

As the target intensity L_(TRGT) of the LED light source 202 isdecreased towards the low-end intensity L_(LE) (and the on times T_(ON)of the drive control signals V_(DRIVE1), V_(DRIVE2) get smaller), theparasitic of the forward converter 140 (i.e., the parasitic capacitancesC_(P1), C_(P2) of the FETs, the parasitic capacitance C_(P3) of theprimary winding of the transformer 220, and other parasitic capacitancesof the circuit) can cause the magnitude of the primary voltage V_(PRI)to slowly decrease towards zero volts after the FETs Q210, Q212 arerendered non-conductive.

FIG. 5 shows example waveforms illustrating the operation of a forwardconverter and a current sense circuit (e.g., the forward converter 240and the current sense circuit 260) when the target intensity L_(TRGT) isnear the low-end intensity L_(LE). The gradual drop off in the magnitudeof the primary voltage V_(PRI) allows the primary winding to continue toconduct the primary current I_(PRI), such that the transformer 220continues to delivered power to the secondary winding after the FETsQ210, Q212 are rendered non-conductive as shown in FIG. 5. In addition,the magnetizing current I_(MAG) continues to increase in magnitude.Accordingly, the controller 150 increases the signal-chopper timeT_(CHOP) (during which the signal-chopper control signal V_(CHOP) islow) by an offset time T_(OS) when the target intensity L_(TRGT) of theLED light source 202 is near the low-end intensity L_(LE). Thecontroller may adjust the value of the offset time T_(OS) as a functionof the target intensity L_(TRGT) of the LED light source 202 as shown inFIG. 6. For example, the controller may adjust the value of the offsettime T_(OS) linearly with respect to the target intensity L_(TRGT) whenthe target intensity L_(TRGT) is below a threshold intensity L_(TH)(e.g., approximately 10%) as shown in FIG. 5.

FIG. 7 is a simplified flowchart of a control procedure 300 executedperiodically by a controller (e.g., the controller 150 of the LED driver100 shown in FIG. 1 and/or the controller controlling the forwardconverter 240 and the current sense circuit 260 shown in FIG. 2). Thecontroller may execute the control procedure 300, for example, at theoperating period T_(OP) of the inverter voltage V_(INV) of the forwardconverter 240. First, the controller determines the appropriate on timeT_(ON) for the drive control signals V_(DRIVE1), V_(DRIVE2) in responseto the target intensity L_(TRGT) and the load current feedback signalV_(I-LOAD) at step 310. If the controller should presently control thehigh-side FET Q210 at step 312, the controller drives the first drivecontrol signal V_(DRIVE1) high to approximately the supply voltageV_(CC) for the on time T_(ON) at step 314. If the target intensityL_(TRGT) is greater than or equal to the threshold intensity L_(TH) atstep 316, the controller 150 sets the signal-chopper time T_(CHOP) equalto the on time T_(ON) at step 318. If the target intensity L_(TRGT) isless than the threshold intensity L_(TH) at step 316, the controllerdetermines the offset time T_(OS) in response to the target intensityL_(TRGT) at step 320 (e.g., using the relationship shown in FIG. 6), andsets the signal-chopper time T_(CHOP) equal to the sum of the on timeT_(ON) and the offset time T_(OS) at step 322.

Next, the controller drives the signal-chopper control signal V_(CHOP)low towards circuit common for the signal-chopper time T_(CHOP) at step324. The controller then samples the averaged load current feedbacksignal V_(I-LOAD) at step 326 and calculates the magnitude of the loadcurrent I_(LOAD) using the sampled value at step 328, for example, usingthe following equation:

$\begin{matrix}{{I_{LOAD} = \frac{n_{TURNS} \cdot V_{I\text{-}{LOAD}} \cdot T_{HC}}{R_{SENSE} \cdot \left( {T_{CHOP} - T_{DELAY}} \right)}},} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$where T_(DELAY) is the total delay time due to the turn-on time and theturn-off time of the FETs Q210, Q212, e.g.,T_(DELAY)=T_(TURN-ON)−T_(TURN-OFF), which may be equal to approximately200 μsec. Finally, the control procedure 300 exits after the magnitudeof the load current I_(LOAD) has been calculated. If the controllershould presently control the low-side FET Q210 at step 312, thecontroller drives the second drive control signal V_(DRIVE2) high toapproximately the supply voltage V_(CC) for the on time T_(ON) at step330, and the control procedure 300 exits without the controller drivingthe signal-chopper control signal V_(CHOP) low.

Alternatively, the controller can use a different relationship todetermine the offset time T_(OS) throughout the entire dimming range ofthe LED light source (i.e., from the low-end intensity L_(LE) to thehigh-end intensity L_(HE)) as shown in FIG. 8. For example, thecontroller could use the following equation:

$\begin{matrix}{{T_{OS} = \frac{\frac{V_{BUS}}{4} \cdot C_{PARASITIC}}{{\frac{T_{ON} + T_{{OS}\text{-}{PREV}}}{T_{HC}} \cdot I_{{MAG}\text{-}{MAX}}} + {\frac{K_{RIPPLE}}{n_{TURNS}} \cdot I_{LOAD}}}},} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$where T_(OS-PREV) is the previous value of the offset time, K_(RIPPLE)is the dynamic ripple ratio of the output inductor current I_(L) (whichis a function of the load current I_(LOAD)) i.e.,K _(RIPPLE) =I _(L-PK) /I _(L-AVG),  (Equation 4)and C_(PARASITIC) is the total parasitic capacitance between thejunction of the FETs Q210, Q212 and circuit common.

As previously mentioned, the controller increases and decreases the ontimes T_(ON) of the drive control signals V_(DRIVE1), V_(DRIVE2) forcontrolling the FETs Q210, Q212 of the forward converter 140 torespectively increase and decrease the intensity of the LED lightsource. Due to hardware limitations, the controller may be operable toadjust the on times T_(ON) of the drive control signals V_(DRIVE1),V_(DRIVE2) by a minimum time step T_(STEP), which results in acorresponding step I_(STEP) in the load current I_(LOAD). Near thehigh-end intensity L_(HE), this step I_(STEP) in the load current LOADmay be rather large (e.g., approximately 70 mA). Since it is desirableto adjust the load current I_(LOAD) by smaller amounts, the controlleris operable to “dither” the on times T_(ON) of the drive control signalsV_(DRIVE1), V_(DRIVE2), e.g., change the on times between two valuesthat result in the magnitude of the load current being controlled to DCcurrents on either side of the target current I_(TRGT).

FIG. 9 shows an example waveform of a load current conducted through anLED light source (e.g., the load current I_(LOAD)). For example, theload current I_(LOAD) shown in FIG. 9 may be conducted through the LEDlight source when the target current I_(TRGT) is at a steady-state valueof approximately 390 mA. A controller (e.g., the controller 150 of theLED driver 100 shown in FIG. 1 and/or the controller controlling theforward converter 240 and the current sense circuit 260 shown in FIG. 2)may control a forward converter (e.g., the forward converter 140, 240)to conduct the load current I_(LOAD) shown in FIG. 9 through the LEDlight source. The controller adjusts the on times T_(ON) of the drivecontrol signals V_(DRIVE1), V_(DRIVE2) to control the magnitude of theload current LOAD to between two DC currents I_(L-1), I_(L-2) that areseparated by the step I_(STEP) (e.g., approximately 350 mA and 420 mA,respectively). The load current I_(LOAD) is characterized by a ditheringfrequency f_(DITHER) (e.g., approximately 2 kHz) and a dithering periodT_(DITHER) as shown in FIG. 9. For example, a duty cycle DC_(DITHER) ofthe load current I_(LOAD) may be approximately 57%, such that theaverage magnitude of the load current I_(LOAD) is approximately equal tothe target current I_(TRGT) (e.g., 390 mA for the example of FIG. 9).

FIG. 10 shows an example waveform of the load current LOAD when thetarget current I_(TRGT) is being increased with respect to time. Asshown in FIG. 10, the controller 150 is able to adjust the on timesT_(ON) of the drive control signals V_(DRIVE1), V_(DRIVE2) to controlthe magnitude of the load current I_(LOAD) to between two DC currentsI_(L-1), I_(L-2) that are separated by the step I_(STEP). The duty cycleDC_(DITHER) of the load current I_(LOAD) increases as the target currentI_(TRGT) increases. At some point, the controller is able to control theon times T_(ON) of the drive control signals V_(DRIVE1), V_(DRIVE2) toachieve the desired target current I_(TRGT) without dithering the ontimes, thus resulting in a constant section 400 of the load currentI_(LOAD). As the target current I_(TRGT) continues to increase after theconstant section 400, the controller is able to control the on timesT_(ON) of the drive control signals V_(DRIVE1), V_(DRIVE2) to dither themagnitude of the load current I_(LOAD) between the DC current I_(L-2)and a larger DC current I_(L-3).

However, the constant section 400 of the load current I_(LOAD) as shownin FIG. 10 may cause the human eye to detect a visible step in theadjustment of the intensity of the LED light source. Therefore, when thecontroller is actively adjusting the intensity of the LED light source,the controller is operable to add a periodic supplemental signal (e.g.,a ramp signal I_(RAMP) or sawtooth waveform) to the target currentI_(TRGT). FIG. 11 shows example waveforms of the ramp signal I_(RAMP)and the resulting load current I_(LOAD) when the ramp signal is added tothe target current I_(TRGT). Note that these waveforms are not to scaleand the ramp signal I_(RAMP) is a digital waveform. The ramp signalI_(RAMP) is characterized by a ramp frequency f_(RAMP) (e.g.,approximately 238 Hz) and a ramp period T_(RAMP). The ramp signalI_(RAMP) may have, for example, a maximum ramp signal magnitudeI_(RAMP)-MAX of approximately 150 mA. The ramp signal I_(RAMP) and mayincrease with respect to time in, for example, approximately 35 stepsacross the length of the ramp period T_(RAMP). When the controller addsthe ramp signal I_(RAMP) to the target current I_(TRGT) to control theon times T_(ON) of the drive control signals V_(DRIVE1), V_(DRIVE2), theresulting load current LOAD has a varying magnitude as shown in FIG. 11.As a result, the perception to the human eye of the visible steps in theintensity of the LED light source as the controller is activelyadjusting the intensity of the LED light source is reduced.

When the target current I_(TRGT) returns to a steady-state value, thecontroller may stop adding the ramp signal I_(RAMP) to the targetcurrent I_(TRGT). For example, the controller may decrease the magnitudeof the ramp signal I_(RAMP) from the maximum ramp signal magnitudeI_(RAMP-MAX) to zero across a period of time after the target currentI_(TRGT) has reached a steady-state value.

While FIG. 11 shows the ramp signal I_(RAMP) (i.e., a sawtooth waveform)that is added to the target current I_(TRGT), other periodic waveformscould be used.

Although the present disclosure has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It ispreferred, therefore, that the present disclosure be limited not by thespecific disclosure herein, but only by the appended claims.

What is claimed is:
 1. A load control device for controlling an amount of power delivered to an electrical load, the load control device comprising: a transformer comprising a primary winding configured to conduct a primary current and a secondary winding adapted to supply current to the electrical load; at least one semiconductor switch electrically coupled to generate a primary voltage across the primary winding of the transformer to cause the transformer to transfer power to the secondary winding when the at least one semiconductor switch is conductive; a current sense circuit configured to receive a sense voltage representative of a magnitude of the primary current conducted through the primary winding of the transformer, the current sense circuit comprising an averaging circuit configured to generate a load current signal that is representative of a real component of the primary current by averaging the sense voltage via the averaging circuit when the at least one semiconductor switch is conductive; and a controller configured to generate at least one drive signal for periodically rendering the at least one semiconductor switch conductive for an on time to control a load current conducted through the electrical load in response to the load current signal, the controller further configured to control the current sense circuit to provide the sense voltage to the averaging circuit for the on time plus an additional amount of time to generate the load current signal when a target amount of power to be delivered to the electrical load is less than a threshold amount.
 2. The load control device of claim 1, wherein the additional amount of time is a function of the target amount of power.
 3. The load control device of claim 1, wherein the additional amount of time increases linearly as the target amount of power decreases.
 4. The load control device of claim 1, wherein the controller is configured to control the current sense circuit to provide the sense voltage to the averaging circuit for the on time when the target amount of power to be delivered to the electrical load is greater than the threshold amount.
 5. The load control device of claim 1, wherein the controller is configured to generate a control signal for causing the current sense circuit to provide the sense voltage to the averaging circuit.
 6. The load control device of claim 5, wherein the controller is configured to sample the load current signal when the current sense circuit is being controlled to provide the sense voltage to the averaging circuit.
 7. The load control device of claim 1, wherein the current sense circuit comprises a sense-circuit semiconductor switch configured to disconnect the sense voltage from the averaging circuit, the controller configured to control the sense-circuit semiconductor switch to provide the sense voltage to the averaging circuit when the at least one semiconductor switch is conductive.
 8. The load control device of claim 7, wherein the sense voltage is coupled to the averaging circuit through two series-connected resistors, the sense-circuit semiconductor switch coupled between the junction of the two resistors and a circuit common to allow the sense voltage to be provided to the averaging circuit when the sense-circuit semiconductor switch is non-conductive.
 9. The load control device of claim 1, wherein the at least one semiconductor switch comprises first and second semiconductor switches electrically coupled in series, the controller configured to control the first and second semiconductor switches to generate the primary voltage across the primary winding of the transformer, the transformer configured to transfer power to the secondary winding when either of the first and second semiconductor switches is conductive.
 10. The load control device of claim 9, further comprising: a capacitor electrically coupled between the junction of the first and second semiconductor switches and the primary winding of the transformer to cause the primary voltage across the primary winding to have a positive polarity when the first semiconductor switch is conductive and a negative polarity when the second semiconductor switch is conductive.
 11. The load control device of claim 1, further comprising: a sense resistor coupled in series with the primary winding of the transformer, the sense resistor configured to conduct the primary current and produce the sense voltage.
 12. A light-emitting diode (LED) driver for controlling an intensity of an LED light source, the LED driver comprising: a transformer comprising a primary winding configured to conduct a primary current and a secondary winding adapted to supply current to the LED light source; at least one semiconductor switch electrically coupled to generate a primary voltage across the primary winding of the transformer to cause the transformer to transfer power to the secondary winding when the at least one semiconductor switch is conductive; a current sense circuit configured to receive a sense voltage representative of a magnitude of the primary current conducted through the primary winding of the transformer, the current sense circuit configured to generate a load current signal that is representative of a real component of the primary current by averaging the sense voltage when the at least one semiconductor switch is conductive; and a controller configured to periodically render the at least one semiconductor switch conductive for an on time to control a load current conducted through the LED light source in response to the load current signal, the controller further configured to control the current sense circuit to average the sense voltage for the on time plus an additional amount of time to generate the load current signal when a target amount of power to be delivered to the LED light source is less than a threshold amount.
 13. The LED driver of claim 12, wherein the at least one semiconductor switch comprises first and second semiconductor switches electrically coupled in series, the controller configured to control the first and second semiconductor switches to generate the primary voltage across the primary winding of the transformer, the transformer configured to transfer power to the secondary winding when either of the first and second semiconductor switches is conductive.
 14. The LED driver of claim 13, further comprising: an isolated forward converter comprising the transformer and the first and second semiconductor switches, the isolated forward converter configured to receive a bus voltage and to conduct the load current through the LED light source.
 15. The LED driver of claim 14, wherein the isolated forward converter further comprises an energy-storage inductor operatively coupled in series with the secondary winding of the transformer, the energy-storage inductor comprising a partially-gapped magnetic core set.
 16. The LED driver of claim 13, further comprising: a capacitor electrically coupled between the junction of the first and second semiconductor switches and the primary winding of the transformer to cause a primary voltage across the primary winding to have a positive polarity when the first semiconductor switch is conductive and a negative polarity when the second semiconductor switch is conductive.
 17. The LED driver of claim 12, wherein the additional amount of time is a function of the target amount of power.
 18. The LED driver of claim 12, wherein the additional amount of time increases linearly as the target amount of power decreases.
 19. The LED driver of claim 12, wherein the controller is configured to average the sense voltage for the on time when the target amount of power to be delivered to the LED light source is greater than the threshold amount.
 20. The LED driver of claim 12, wherein the current sense circuit comprises an averaging circuit configured to generate the load current signal when the at least one semiconductor switch is conductive, the current sense circuit further comprising a sense-circuit semiconductor switch configured to disconnect the sense voltage from the averaging circuit, the controller configured to control the sense-circuit semiconductor switch to provide the sense voltage to the averaging circuit when the at least one semiconductor switch is conductive. 